World’s Soil Resources

Soil management has a considerable effect on how the soil may fulfil its ecosystem services. Mineral
and organic fertilizer may compensate for poor inherent nutrient conditions in a soil; drainage may remedy
excessive wetness in soils, or leach salts when these are present; amendments (lime or gypsum) may correct
very acid or highly sodic soils. However, these interventions always have a cost in terms of labour and inputs,
and they may also have negative side effects, such as groundwater contamination.
In this section a number of soil qualities essential for the provision of ecosystem services are discussed and
related to the major soil groups summarized and illustrated in Annex A 35.
3.6.1 | Inherent soil fertility
The capability of a soil to provide sufficient nutrients to crops, grasses and trees is a major quality of soils
that supports all provisioning services of the ecosystem. Sixteen nutrients are essential for plant growth and
living organisms in the soil. These fall into two different categories: macronutrients and micronutrients.
Macronutrients are the most important nutrients for plant development and relatively high quantities
are required. Macronutrients include: carbon (C), oxygen (O), hydrogen (H), nitrogen (N), phosphorus (P),
potassium (K), calcium (Ca), magnesium (Mg), and sulphur (S). Micronutrients, on the other hand, are needed
in smaller amounts, but are still crucial for plant development and growth. Micronutrients include iron
(Fe), zinc (Zn), manganese (Mn), boron (B), copper (Cu), molybdenum (Mo) and chlorine (Cl). Nearly all plant
nutrients are taken up in ionic forms from the soil solution as cations or as anions.
Soil properties directly related to the amount and availability of nutrients in the soil are: (i) soil texture
(clayey soils contain more nutrients than sandy ones); (ii) the type of clay minerals present (smectitic clays
absorb more ions than kaolinitic ones); (iii) the soil organic carbon content (more SOC corresponds with a
larger amount of nutrients); and (iv) the cation exchange capacity that corresponds to the total of Ca, Mg,
K, Na (basic ions) and Al and H (acidic ions) exchangeable with the soil solution. A large amount of available
nutrients is present in Vertisols, Chernozems (Borolls), Kastanozems (Ustolls) and Phaeozems (Udolls). Also
volcanic soils (Andosols) and alluvial soils (Fluvisols/Fluvents) generally have a large nutrient content. On
the other hand, sandy soils (Arenosols/Psamments) and highly leached soils (Ferralsols/Oxisols and Acrisols/
Ultisols) generally have a small nutrient content.
The amount of nutrients that a soil can provide to plants within the growing season represents a limit to
nutrient mining. Nutrient mining occurs when crops take out a high proportion of the nutrients available in
the soil, leaving a nutrient imbalance that threatens the sustained provision of food and ecosystem services.
These challenges are discussed in Section 6.8. Figure 3.1 illustrates an estimation of the nutrient availability in
soils globally based on information contained in HWSD.
Soil depth to a hard or an impermeable layer is a vital factor that determines the capability of roots to take
hold and determines the total volume of nutrients and water available to crops and vegetation. Soils tend to
be deeper when strong weathering conditions prevail over a long period and wherever the parent material is
readily weathered. Typical soils include Ferralsols and Nitisols). Shallow soils often occur in mountainous areas
(Leptosols) and in dry areas characterized by indurated layers of silica, calcium carbonate or gypsum (Durisols/
Durids, Calcisols/Calcids and Gypsisols/Gypsids). Each plant type has its own ideal rooting conditions. Tubers
are the most sensitive to soil depth and volume limitations (Fischer et al., 2008; Grossnickle, 2005; Unger
and Kaspar, 1994; McSweeney and Jansen, 1984; Myers et al., 2007). Figure 3.2 illustrates global soil rooting
conditions
Status of the World’s Soil Resources | Main Report Global Soil Resources
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